Compositing Shadow pass problem
The reason for jagged shadows lies in the technique used for rendering the shadows.
I assume you used the blender internal render engine and the standard shadow of the lamp; for spots lamps the default is the buffered shadow.

Here is a rough explanation how buffer shadows are calculated

With buffer shadows something like a render is made from the view of the light source lamp. Every surface point not visible on this lamp-rendered image isn't reachable by this light source. Thus, these pixels of the lamp-rendered image projected to the unreachable surfaces will be a good approximation of the shadow in the final render. According to this the blockyness depends on the resolution of this lamp-rendered image. It can be adjusted at the lamp settings:



Here is more info on how the buffering works.

So increasing the size is the solution?

No. A better way is to change the shadow rendering technique. The other one available in blender internal is Ray Shadow which probably is the best solution.


I hope your shadow pass is blockless with this method now :)